A Skyscraper is another single-digit pattern — like a bent X-Wing with a metaphor that sticks.
All it needs is a floor, sturdy walls, and a steep roof. Then you can eliminate candidates that see both roof cells.
It is also the shortest useful AIC / X-Chain shape: strong–weak–strong.
Before this: Learn X-Wing first (same single-digit idea).
Why it works
The floor is a weak link: two cells in the same row (level floor). The walls are strong bi-local columns (exactly two candidates). The roof ends are not on one row — otherwise it would be an X-Wing.
On each strong wall, one end is true and the other is false for that digit. The shared base cannot hold two truths. So at least one rooftop must be true.
Any cell that can see both rooftops is attacked no matter which rooftop is true. That cell cannot be the digit.
How to spot it
Look for floor, walls, and a steep roof. Rotate/flip — same pattern sideways or upside-down.
Demonstration
For one digit, find two strong links (exactly two candidates on a line) — these will be the walls.
Align them on a level floor (weak link in one row). The other ends stick up as a steep roof. The green cells form a Skyscraper on 9.
This is strong–weak–strong — a tiny X-Chain / AIC. One of the two rooftops must be 9.
Eliminate 9 from cells that see both rooftops (red). Both worlds sting those cells.
Example 1
Find floor + walls + steep roof on one digit (two strong links sharing a base).
This Skyscraper on 1 has two rooftops.
Remove 1 from cells that see both rooftops (red). At least one roof must be true.
Example 2
Find floor + walls + steep roof on one digit (two strong links sharing a base).
This Skyscraper on 2 has two rooftops.
Remove 2 from cells that see both rooftops (red). At least one roof must be true.